As is known in the prior art, load beam transducers optimally function in pure axial tension and compression. In certain applications, off axis loads are involved. These loads can result in inaccurate measurements or could, in fact, damage the beam. Load beams have been employed in many applications. As known in the prior art, side or transverse loads instead of axial loads can cause false reactions or fracture the beam. It is desirable to substantially reduce or eliminate side load effects. It is a further desire to provide another linkage for the beam in the event it does fail.
A load beam is typically small in size, for example, 0.085 inches wide and 0.270 inches thick. The length can be between 0.3 to 1 inch or more. The load beam basically is a relatively thin platform and is the active sensing element where the beam responds to axial tension and compression forces to cause gages or sensors located on the beam to produce an output indicative of the magnitude of the applied force. The beam is designed for maximum micro strain on the gauges located on the beam at maximum rated load. In tension or compression, the load beam can accept off axis perturbations to about 36 inch pounds before yielding and deforming the beam. These loads will be encountered especially in the compression mode, where beam-buckling can also occur. In any event, the critical buckling load is 16 times less, without side support for guiding the load beam in compression.
Co-pending, commonly-owned U.S. patent application Ser. No. 10/396,241, entitled, “A Load Beam Apparatus to Prevent Improper Operation Due to Off-Axis Loads,” filed on Mar. 25, 2003, teaches a linear bearing surrounding the beam which protects the load beam from buckling as a result of side or off-axis forces. However, the use of the linear bearing described in the aforementioned patent increases the cost and overall size of the load cell.
One simple remedy to overcome the reduced buckling load along a specific direction is to increase the load beam size in that direction. However, this is disadvantageous as it causes other problems. As one skilled in the art would recognize, the ability of the load-beam to measure stress is reduced by one-half when the cross-sectional area is doubled. Furthermore, as strain is proportional to stress, a reduction by one-half of the stress reduces the strain or deflection by one-half. Finally, as strain is proportional to accuracy, a reduction of one-half of the strain causes a reduction of one-half of the output voltage. Thus, by doubling the cross-sectional shape to increase off-axis force resistance, the measurement accuracy is significantly reduced. For example, doubling the cross-sectional area of a load beam designed to measure a maximum load of 500 lbs. on a 500 lb. scale would result in a measurement device that measures the 500 lb. load on a 1000 lb. scale. Hence, the accuracy of the measurement is reduced.
Accordingly, an improved system is needed that would increase the ability of the beam transducer to withstand excessive side or off-axis forces while maintaining reasonable measurement accuracy.